Our aim is to clarify the background to the growth of the Mongol empire. The view that aspects of the environment such as climate are related to the rises of dynasties has existed for quite some time. There is also a possibility that the climate in the beginning of the 13th century was largely involved in the sudden rise to power of the Mongol empire. A prominent hypothesis in research to date was that Chinggis Khan unified the Mongolian people, amassed power, and set out for world conquest during a period of cooling after the Medieval Warm Epoch, in which there was warming on a global scale. We examined the hypothesis by using the paleo-environmental methods. Our results indicate that the regional climate during the rise of Chinggis khan was cold and dry, and during the conquests was warm and wet. We think that the reason for the Mongol empire's expansion may be hidden in the Mongolian Plateau's successful farming and stock raising due to its tendency toward warming and humidity.